Beamforming is a technique by which an array of antennas can be steered to transmit radio signals in a specific direction. Rather than simply broadcasting energy/signals in all directions, the antenna arrays that use beamforming, determine the direction of interest and send/receive a stronger beam of signals in that specific direction.
In this technique, each antenna element is fed separately with the signal to be transmitted. The phase and amplitude of each signal is then added constructively and destructively in such a way that they concentrate the energy into a narrow beam or lobe.
The more radiating elements that make up the antenna, the narrower the beam. An artifact of beamforming is side lobes.
The side lobes are essentially radiation of a signal that is not intended to form the main beam, but instead spreads out in various directions.
If an antenna array is poorly designed, it can cause excessive interference from these side lobes of a beamformed signal.